I'm tempted to say that it is because Toronto fans have had so much practice supporting losing teams, but in reality I have nothing but respect for the way they support their teams.
All teams will experience a drop-off of support when they go through long losing spells because their fan bases are made up of hard-core fans who will always go to support their team, and casual fans who will jump on and off the bandwagon at will.
With Toronto's large population base, they seem to have enough of the hard core fans to ride out the lean years in both hockey and soccer. Could Ottawa do that with MLS? There is really only one way to tell and that is to put a team here and see. My feeling is that they could, based on the explosive growth in soccer here, and the large number of ex-pat Europeans based here.